Distributed power generation is becoming more common as society increases its use of renewable resources such as solar energy. The majority of solar power systems currently in use employ multiple polycrystalline silicon photovoltaic panels connected to string inverters that send the generated power back into an electric utility grid. The photovoltaic panels are usually connected in series to provide a high direct current (DC) voltage to the string inverters. Other solar power systems use micro-inverters, each of which is connected to a single photovoltaic panel, which allows for low voltage DC connections and individual tracking of the power output of each panel.
The proliferation of distributed power generation has caused some problems in maintaining the stability of the utility grid. As a result, many electric utilities world-wide are seeking increased functionality in grid-tied inverters to help stabilize the grid. For example, some require grid-tied photovoltaic inverters to gradually reduce output power when the grid frequency deviates beyond a set limit. Furthermore, to increase the safety of utility systems, most photovoltaic systems are required to have separate voltage-frequency relays which monitor the voltage and the frequency of the grid and disconnect all photovoltaic inverters from the grid if the voltage or frequency deviates beyond a predetermined range. Voltage-frequency relays provide additional assurance that photovoltaic inverters do not push power onto the utility grid when the grid is disconnected. However, voltage-frequency relays add cost to photovoltaic systems beyond what would otherwise be required.
Photovoltaic inverters include grid-tied inverters and stand-alone inverters. Grid-tied inverters act as a current source and can only be used to pump power into the utility grid. Stand-alone inverters act as a voltage source and can provide power to a load in the absence of an electric utility grid. For a number of reasons, grid-tied inverters are far more prevalent than stand-alone inverters, and therefore the costs of production substantially favors grid-tied inverters. Furthermore, grid-tied inverters are sold based on a pay-back of savings in utility bills, often coupled with financial incentives by local governments to encourage the use of “green” energy. Stand-alone inverters, on the hand, usually have no pay-back and no financial incentives, and function only to provide power in the rare event of a grid outage or to provide power in places that have no utility grid. However, many customers would prefer a system that can provide both grid-tied capability and backup power in the event of an outage, as long as the cost of the final system is not substantially higher than that of the grid-tied system alone.
As distributed power generation systems proliferate, utilities are selling less average power to the grid, but they must still provide the same peak grid load because distributed power generation systems usually cannot be relied on to always provide power. As a result, some utilities are trying to cut costs by enforcing peak load requirements on their customers. The local peak power reductions can be accomplished through the smart control of loads or through local battery or other storage that maintains power during the peak load times. Utilities also expect distributed power generation systems to provide additional functionality for producing local volt-ampere reactive (VAR) control on the grid. Utilities would also prefer to cut costs caused by the distorted power factor of customer loads. Incorporating these functions into grid-tied inverters can increase cost. Relatedly, grid-tied battery systems can level loads. However, such systems are usually large (utility) scale because small residential-sized systems are rarely cost-effective for use merely as load-leveling systems.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.